Add SPDX-License-Identifier to source files

SPDX License List is a list of (common) open source
licenses that can be referred to by a “short identifier”.
It has several advantages compared to the common "license header texts"
usually found in source files.

Some of the advantages:
* It is precise; there is no ambiguity due to variations in license header
  text
* It is language neutral
* It is easy to machine process
* It is concise
* It is simple and can be used without much cost in interpreted
  environments like java Script, etc.
* An SPDX license identifier is immutable.
* It provides simple guidance for developers who want to make sure the
  license for their code is respected

See http://spdx.org for further reading.

Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>

Closes: #1439
Approved by: cgwalters

src/libostree/ostree-repo-commit.c

@@ -1,6 +1,8 @@
 /*
  * Copyright (C) 2011,2013 Colin Walters <walters@verbum.org>
  *
+ * SPDX-License-Identifier: LGPL-2.0+
+ *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
  * License as published by the Free Software Foundation; either